Pharmacokinetics of intrathecally applied BDNF and effects on spinal motoneurons

Changes in truncated trkB and p75 receptor expression in the rat spinal cord following spinal cord hemisection and spinal cord hemisection plus neurotrophin treatment

Although numerous studies have examined the effects of neurotrophin treatment following spinal cord injury, few have examined the changes that occur in the neurotrophin receptors following either such damage or neurotrophin treatment. To determine what changes occur in neurotrophin receptor expression following spinal cord damage, adult rats received a midthoracic spinal cord hemisection alone or in combination with intrathecal application of brain-derived neurotrophic factor (BDNF) or neurotrophin-3 (NT-3). Using immunohistochemical and in situ hybridization techniques, p75, trkA, trkB, and trkC receptor expression was examined throughout the spinal cord. Results showed that trkA, full-length trkB, and trkC receptors were not present in the lesion site but had a normal expression pattern in uninjured parts of the spinal cord. In contrast, p75 receptor expression occurred on Schwann cells throughout the lesion site. BDNF and NT-3 (but not saline) applied to the lesion site increased this expression. In addition, the truncated trkB receptor was expressed in the border between the lesion and intact spinal cord. Truncated trkB receptor expression was also increased throughout the white matter ipsilateral to the lesion and BDNF (but not NT-3 or saline) prevented this increase. The study is the first to show changes in truncated trkB receptor expression that extend beyond the site of a spinal cord lesion and is one of the first to show that BDNF and NT-3 affect Schwann cells and/or p75 expression following spinal cord damage. These results indicate that changes in neurotrophin receptor expression following spinal cord injury could influence the availability of neurotrophins at the lesion site. In addition, neurotrophins may affect their own availability to damaged neurons by altering the expression of the p75 and truncated trkB receptor.

Epi-arachnoidal drug deposit: a rare complication of intrathecal drug therapy

Intrathecal (i.t.) drug application is accepted as a highly effective treatment option for various neurological conditions. Technical risks and potentially dangerous complications require appreciation. We present the case of a patient treated with i.t. recombinant, human brain-derived neurotrophic factor (rhBDNF) as an experimental therapy for amyotrophic lateral sclerosis (ALS). Five days after starting the i.t. drug infusion, she complained of severe headache and nausea. Radiological studies suggested the catheter was located within the epi-arachnoidal space. A deposit of more than 10 ml secluded from the subarachnoidal space was found within this space. I.t. contained a high concentration of the applied drug. Revision of the catheter resulted in complete recovery from symptoms and i.t. infusion could be continued. The epi-arachnoidal positioning of a spinal catheter is a potential cause for treatment failure. If the membrane around the fluid deposit ruptures, the drug could be released into the subarachnoidal space, with the consequence of a potentially life-threatening complication.

Neurotrophic factors and neuro-muscular disease: II. GDNF, other neurotrophic factors, and future directions

This is the second of two reviews in which we discuss the essential aspects of neurotrophic factor neurobiology, the characteristics of each neurotrophic factor, and their clinical relevance to neuromuscular diseases. The previous paper reviewed the neurotrophin family and neuropoietic cytokines. In the present article, we focus on the GDNF family and other neurotrophic factors and then consider future approaches that may be utilized in neurotrophic factor treatment.

Distribution and retrograde transport of trophic factors in the central nervous system: functional implications for the treatment of neurodegenerative diseases

Neurotrophins play a crucial role in the maintenance, survival and selective vulnerability of various neuronal populations within the normal and diseased brain. Several families of growth promoting substances have been identified within the central nervous system (CNS) including the superfamily of nerve growth factor related neurotrophin factors, glial derived neurotrophic factor (GDNF) and ciliary neurotrophic factor (CNTF). In addition, other non-neuronal growth factors such as fibroblast growth factor (FGF) have also been identified. This article reviews the trophic anatomy of these factors within the CNS. Intraventricular and intraparenchymal injections of exogenous nerve growth factor result in retrograde labeling mainly within the cholinergic basal forebrain. Distribution of brain derived neurotrophic factor (BDNF) following intraventricular injection is minimal due to the binding to the trkB receptor along the ventricular wall. In contrast, intraparenchymal injections of BDNF results in widespread retrograde transport throughout the CNS. BDNF has also been shown to be transported anterogradely within the CNS. Infusion of GDNF into the CNS results in retrograde transport limited to the nigrostriatal pathway. Hippocampal injections of NT-3 retrogradely label mainly basal forebrain neurons. Retrograde transport of radiolabeled CNTF has only been observed in sensory neurons of the sciatic nerve. Following intraventricular and intraparenchymal infusion of radiolabeled bFGF, retrograde neuronal labeling was found in the telecephalon, diencephalon, mesencephalon and pons. In contrast retrograde labeling for aFGF was found only in the hypothalamus and midbrain. Since select neurotrophins traffic anterogradely and retrogradely within the nervous system, these proteins could be used to treat neurological diseases such as Alzheimer's disease, Parkinson's disease and amyotrophic lateral sclerosis.

Acetylcholinesterase gene expression in axotomized rat facial motoneurons is differentially regulated by neurotrophins: correlation with trkB and trkC mRNA levels and isoforms

We examined the potential influences of muscle-derived neurotrophins on the acetylcholinesterase (AChE) gene expression of adult rat motoneurons. Seven days after facial nerve transection, both AChE mRNA and enzyme activity levels were markedly reduced in untreated and vehicle-treated facial motoneurons, suggesting positive regulation of motoneuron AChE expression by muscle-derived factors. Because skeletal muscle is a source of neurotrophin-3 (NT-3), NT-4/5, and BDNF, these neurotrophins were individually infused onto the proximal nerve stump for 7 d, beginning at the time of axotomy. The trkB ligands NT-4/5 and BDNF prevented the downregulation of AChE mRNA and enzymatic activity, as determined by in situ hybridization, biochemical assay, and histochemical visualization of enzyme activity. In contrast, NT-3 had limited effects, and NGF was without effect. Because motoneurons normally express both trkB and trkC receptors and the trkC ligand NT-3 is the most abundant muscle-derived neurotrophin, we investigated possible reasons for the limited effects of NT-3. In situ hybridization and reverse transcription-PCR both revealed a downregulation of trkC mRNA in axotomized motoneurons, which contrasted the upregulation of trkB expression. Furthermore, isoforms of trkC were detected carrying insertions within their kinase domains, known to limit certain trkC-mediated signal transduction pathways. Because the changes in trkB and trkC mRNA levels were not significantly altered by neurotrophin infusions, it is unlikely they were induced by loss of muscle-derived neurotrophins. These results demonstrate that NT-4/5 and BDNF stimulate AChE gene expression in motoneurons and support the concept that muscle-derived trkB ligands modulate the cholinergic phenotype of their innervating motoneurons.

Brain-derived neurotrophic factor stimulates hindlimb stepping and sprouting of cholinergic fibers after spinal cord injury

Neurotrophic factors have been proposed as a therapeutic treatment for traumatic brain and spinal cord injury. The present study determined whether exogenous administration of one such factor, brain-derived neurotrophic factor (BDNF), could effect behavioral recovery and/or histopathological changes after spinal cord injury. Adult rats received a mild or moderate contusion injury or complete transection of the mid-thoracic spinal cord. Immediately thereafter, they were infused intrathecally with vehicle or BDNF for 28 days. Behavioral recovery was evaluated for 6 weeks after injury, at which time the rats were sacrificed and the spinal cord tissue was examined histologically. The infusion of BDNF resulted in acute stimulation of hindlimb activity. These effects included activation of alternating airstepping in injured rats when the hindlimbs were unloaded as well as slight improvements in the rate of recovery in open field locomotion score. BDNF infusion was also associated with enhanced growth of cholinergic fibers at the injury epicenter, but did not affect white matter sparing or density of serotonergic axons at or below the injury site. Based on immunohistochemical detection of BDNF protein distribution, these described effects are likely to be mediated by the activation of cells and axons within the central injury region and the along the peripheral rim of the spinal cord. Together, these findings demonstrate that the exogenous infusion of BDNF after spinal trauma can influence postinjury outcome through mechanisms that include acute stimulation of hindlimb activity and neuritogenesis at the injury site.

Adenovirus-mediated transfer of the neurotrophin-3 gene into skeletal muscle of pmn mice: therapeutic effects and mechanisms of action

Several neurotrophic factors (CNTF, BDNF, IGF-1) have been suggested for the treatment of motor neuron diseases. In ALS patients, however, the repeated subcutaneous injection of these factors as recombinant proteins is complicated by their toxicity or poor bioavailability. We have constructed an adenovirus vector coding for neurotrophin-3 (AdNT-3) allowing for stable and/or targeted delivery of NT-3 to motoneurons. The intramuscular administration of this vector was tested in the mouse mutant pmn (progressive motor neuronopathy). AdNT-3-treated pmn mice showed prolonged lifespan, improved neuromuscular function, reduced motor axonal degeneration and efficient reinnervation of muscle fibres. NT-3 protein and also adenovirus vectors, when injected into muscle, can be transported by motoneurons via retrograde axonal transport to their cell bodies in the spinal cord. Using ELISA and RT-PCR analyses in muscle, spinal cord and serum of AdNT-3-treated pmn mice, we have investigated the contribution of these processes to the observed therapeutic effects. Our results suggest that most if not all therapeutic benefit was due to the continuous systemic liberation of adenoviral NT-3. Therefore, viral gene therapy vectors auch as adenoviruses, AAVs, lentiviruses and new types of gene transfer not based on viral vectors that allow for efficient in vivo liberation of neurotrophic factors have potential for the future treatment of human motor neuron diseases.

LIF (AM424), a promising growth factor for the treatment of ALS

Growth factors are theoretically promising agents for ALS therapy, but have been disappointing in subcutaneous delivery due to either toxicity or lack of major efficacy. Leukaemia inhibitory factor (LIF), was named after its effect on haemopoietic cells, and belongs to a group of cytokines which includes CNTF, IL-6, CT-1, OM and IL-11. All group members use the gp130 signal transducing subunit for intracellular signalling, but show differences in biological effect. In vitro and in vivo studies on axotomy and nerve crush models demonstrate a powerful effect of LIF in the survival of both motor and sensory neurones, while reducing denervation induced muscle atrophy. Its effects in muscle also include stimulating myoblast proliferation in vitro, and up-regulation after muscle injury. LIF will also stimulate muscle regeneration in vivo when applied exogenously after injury. In published studies of both axotomy induced neuronal death and in the Wobbler mouse models LIF is active at doses of 10 microg/kg delivered systemically, well below the expected maximum tolerated dose suggested by primate safety studies. LIF is expressed in low levels by spinal cord neurones with significant up-regulation when the neurones are damaged by BOAA toxin, an excitatory amino acid associated with a form of ALS. This augments other evidence suggesting LIF is a trauma factor playing a role in the injury response of adult neuronal tissue, and may be more effective than related growth factors. Taken together, the data suggests LIF is a physiologically relevant trophic factor with implications in clinical medicine as a therapy for ALS, and a human recombinant form (AM424), entered human clinical trials during 1998.

Direct convective delivery of macromolecules to the spinal cord

OBJECT

Because of the limited penetration of macromolecules across the blood-spinal cord barrier, numerous therapeutic compounds with potential for treating spinal cord disorders cannot be used effectively. The authors have developed a technique to deliver and distribute macromolecules regionally in the spinal cord by using convection in the interstitial space.

METHODS

The authors designed a delivery system connected to a "floating" silica cannula (inner diameter 100 microm, outer diameter 170 microm) that provides for constant volumetric inflow to the spinal cord. A solution containing albumin that was either unlabeled or labeled with carbon-14 or gadolinium was infused at various volumes (3, 6, 10, 20, 40, or 50 microl) at a rate of 0.1 microl/minute into the spinal cord dorsal columns of nine swine and into the lateral columns of three primates (Macaca mulatta). Volume of distribution (Vd), concentration homogeneity, and percentage of recovery were determined using scintillation analysis, kurtosis calculation (K), and quantitative autoradiography (six swine), magnetic resonance imaging (one swine and three primates), and histological analysis (all animals). Neurological function was observed for up to 3 days in four of the swine and up to 16 weeks in the three primates. The Vd of 14C-albumin was linearly proportional (R2=0.97) to the volume of infusion (Vi) (Vd/Vi=4.4+/-0.5; [mean+/-standard deviation). The increases in Vd resulting from increases in Vi were primarily in the longitudinal dimension (R2=0.83 in swine; R2=0.98 in primates), allowing large segments of spinal cord (up to 4.3 cm; Vi 50 microl) to be perfused with the macromolecule. The concentration across the area of distribution was homogeneous (K=-1.1). The mean recovery of infused albumin from the spinal cord was 85.5+/-5.6%. Magnetic resonance imaging and histological analysis combined with quantitative autoradiography revealed the albumin infusate to be preferentially distributed along the white matter tracts. No animal exhibited a neurological deficit as a result of the infusion.

CONCLUSIONS

Regional convective delivery provides reproducible, safe, region-specific, and homogeneous distribution of macromolecules over large longitudinal segments of the spinal cord. This delivery method overcomes many of the obstacles associated with current delivery techniques and provides for research into new treatments of various conditions of the spinal cord.

Brain-derived neurotrophic factor fails to arrest neuromuscular disorders in the paralysé mouse mutant, a model of motoneuron disease

Several new neurotrophic factors have been recently identified and shown to prevent motoneuron death in vitro and in vivo. One such agent is brain-derived neurotrophic factor (BDNF). In this study, we tested BDNF on an animal model of early-onset motoneuron disease: the paralysé mouse mutant, characterized by a progressive skeletal muscle atrophy and the loss of 30-35% of spinal lumbar motoneurons between the first and second week post-natal. The results show that subcutaneous injections of 1 or 10 mg/kg BDNF did not have any significant effect in increasing the mean survival time of mutant mice or in preventing the loss of motor function and total body weight in paralysé mice. The weight and choline acetyltransferase activity of specific muscles and the number of motoneurons in the spinal cords were identical in BDNF-treated and placebo-injected paralysé mice. These results suggest that BDNF does not act on the disease process in paralysé mice in the conditions we used. By contrast, BDNF has previously been shown to partially prevent the loss of motor function in the wobbler mouse, a suggested model of later-onset motoneuron disease. Taken together these findings suggest that BDNF acts differently on early and late-onset motoneuron diseases. It is however possible that treatment of paralysé mice with BDNF or combinations of different neurotrophic factors prior to the phenotypical expression of the paralysé mutation may prevent the loss of motor function and motoneurons in mutant mice.

Effects of neurotransplants and BDNF on the survival and regeneration of injured adult spinal motoneurons

We compared the effects of peripheral nerve grafts, embryonic spinal cord transplants and brain-derived neurotrophic factor (BDNF) on the survival and axon regeneration of adult rat spinal motor neurons undergoing retrograde degeneration after ventral root avulsion. Following implantation into the dorsolateral funiculus of the injured spinal cord segment, neither a peripheral nerve graft nor a combination of peripheral nerve graft with embryonic spinal cord transplant could prevent the retrograde motor neuron degeneration induced by ventral root avulsion. However, intrathecal infusion of BDNF promoted long-term survival of the lesioned motor neurons and induced abundant motor axon regeneration from the avulsion zone along the spinal cord surface towards the BDNF source. A combination of ventral root reconstitution and BDNF treatment might therefore be a promising means for the support of both motor neuron survival and guided motor axon regeneration after ventral root lesions.

S100 beta prevents the death of motor neurons in newborn rats after sciatic nerve section

We have examined whether S100 beta rescues axotomized spinal motor neuron death. Animals that had undergone transection of the sciatic nerve at birth were treated either with S100 beta, or the vehicle. The number of surviving motor neurons and the motor neuron diameter was assessed 14 days later. Treatment with S100 beta rescued motor neuron death and preserved the motor neuron diameter in the lesioned side. These results suggest that S100 beta is a neurotrophic factor for motor neurons in vivo and this agent may have therapeutic potential in damaged motor neuron disorder.

Adenoviral gene transfer of ciliary neurotrophic factor and brain-derived neurotrophic factor leads to long-term survival of axotomized motor neurons

The neurotrophic factors ciliary neurotrophic factor and brain-derived neurotrophic factor can prevent motor neuron cell death during development and after nerve lesion in neonatal rodents. However, local and systemic application of these factors to newborn rats with damaged motor nerves rescues motor neurons only transiently during the first two weeks after axotomy. In order to test the effect of continuous delivery of these factors, the effect of localized injection of CNTF- or BDNF-transducing recombinant adenoviruses into the lesioned nerves was investigated. Under such conditions, survival of axotomized motor neurons is maintained for at least 5 weeks. This way of delivery corresponds to the physiological situation in adult rodents, under which endogenous CNTF is present in the cytosol of Schwann cells and BDNF expression is upregulated after nerve lesion, making these factors available to the damaged motor neurons. Recent results show that overexpression of muscle-derived neurotrophin-3 prevents degeneration of axons and motor endplates, but has only little effect on the number of motor neuron cell bodies in a murine animal model of motor neuron disease. Therefore, techniques suitable for tonic exposure to both nerve- and muscle-derived neurotrophic factors may have implications for the design of future therapeutic strategies against human motor neuron disease.

Intraventricular brain-derived neurotrophic factor reduces infarct size after focal cerebral ischemia in rats

Brain-derived neurotrophic factor (BDNF), acting through the high-affinity receptor tyrosine kinase (TrkB), is widely distributed throughout the central nervous system and displays in vitro trophic effects on a wide range of neuronal cells, including hippocampal, cerebellar, and cortical neurons. In vivo, BDNF rescues motorneurons, hippocampal, and substantia nigral dopaminergic cells from traumatic and toxic brain injury. After transient middle cerebral artery occlusion (MCAO), upregulation of BDNF-mRNA in cortical neurons suggests that BDNF potentially plays a neuroprotective role in focal cerebral ischemia. In the current study, BDNF (2.1 micrograms/d) in vehicle or vehicle alone (controls) was delivered intraventricularly for 8 days, beginning 24 hours before permanent middle cerebral artery occlusion by intraluminal suture in Wistar rats (n = 13 per group). There were no differences in physiological variables recorded during surgery for the two groups. Neurological deficit (0 to 4 scale), which was assessed on a daily basis, improved in BDNF-treated animals compared with controls (P < 0.05; analysis of variance and Scheffe's test). There were no significant differences in weight in BDNF-treated animals and controls during the experiment. After elective killing on day 7 after MCAO, brains underwent 2,3,5-triphenyltetrazolium chloride staining for calculation of the infarct volume and for histology (hematoxylin and eosin and glial fibrillary acid protein). The mean total infarct volume was 83.1 +/- 27.1 mm3 in BDNF-treated animals and 139.2 +/- 56.4 mm3 in controls (mean +/- SD; P < 0.01, unpaired, two-tailed t-test). The cortical infarct volume was 10.8 +/- 7.1 mm3 in BDNF-treated animals and 37.9 +/- 19.8 mm3 in controls (mean +/- SD; P < 0.05; unpaired, two-tailed t-test), whereas ischemic lesion volume in caudoputaminal infarction was not significantly different. These results show that pretreatment with intraventricular BDNF reduces infarct size after focal cerebral ischemia in rats and support the hypothesis of a neuroprotective role for BDNF in stoke.